* Iterate over all imported OpenPGP certificates (with the option to skip
further questions). Previously, the question was asked only for the first
certificate imported from each file for files containing at least one
secret key.
* Skip imports of public keys for already known keys, but do not skip
imports of previously unknown public keys because the corresponding secret
key may be on a smart card that's already known.
* Skip certificates that do already have ultimate owner trust.